It is known that the wheels for tires of vehicles consist of a cylindrical rim made of metal having, at the axial extremities, annular retention flanges between which is defined a channel for pressure fitting a tire.
In fitting configuration, the side portions of the tire, so-called “beads”, are stopped up tight against the retention flanges.
Inside the tire, an inner tube can be fitted or, in the case of “tubeless” type tires, air under pressure can be directly introduced.
To perform the tire fitting and removal operations onto and from the relative rims, tire changing machines are currently used which allow removing the tire from the relevant rim, to perform maintenance jobs or replace the inner tube, the rim and/or the tire itself, and then refitting the same tire, or a replacement tire, on the wheel rim.
Traditionally, the tire changing machines are composed of a base frame which supports grip and rotation moving means for the wheel rim being worked upon.
The rotation axis of the grip and rotation moving means can be vertical or horizontal according to the type of tire changing machine.
In a particular type of vertical-axis tire changing machine, for example, the base frame is shaped to define a vertical upright supporting a traditional work tool positionable near a retention flange of the rim to separate and remove the relative tire bead, during the removal phase, as well as to guide the bead inside the channel defined among the retention flanges, during the fitting phase.
Such traditional work tool is composed of a pair of bead guide elements integrated into a single rigid body, made of metal or plastic.
One of these elements, conventionally called “guiding wing”, has a particularly complex shape that reproduces a portion at least in part of truncated cone shape, with a protruding base edge that acts as a locator for the tire bead.
The other element on the other hand has a more or less spherical shape, and because of this is conventionally called a “guiding nut”.
The guiding nut and the guiding wing are arranged substantially away from one another according to a preset geometry.
In an operating configuration, in particular, the traditional work tool is arranged with the guiding wing resting on the rim retention flange, with the base edge following the circumference, at least in part, and with the guiding nut protruding beyond the retention flange.
To allow taking the bead out of the rim channel, the traditional work tool has a horizontal tang extending from the guiding nut on the opposite side to the guiding wing.
During operation, this tang is arranged in the proximity of the flange to act as a resting fulcrum for a manual bead extraction lever.
In fact, during tire removal, the tire changing machine operator inserts a manual lever between the bead and the retention flange and, by levering on the horizontal tang, moves the bead beyond the retention flange until it is above the guiding nut.
In this position, a part of the bead remains under the guiding wing, while another part is placed above the guiding nut. By turning the rim and, if necessary, with the aid of the manual lever, the operator is able to extract the bead from the entire length of the retention flange.
The traditional work tool has various drawbacks tied, in particular, to its not very functional and practical operation and to the fact that the operator has to perform laborious and tiring jobs with the manual lever.
To overcome such drawbacks, different types of automatic turrets are known that replace the previously described traditional work tool.
An example of turret is described in the U.S. Pat. No. 7,128,119, granted Oct. 31, 2006, and corresponding European Patent No. 1,593,533 and comprises a grip tool (4, 4a) designed to grip the tire bead and remove it from the rim in an automatic way, without tiring the operator.
For this purpose, the grip tool is mobile between an insertion configuration and an extraction configuration by means of an articulated quadrilateral mechanism (described in column 3, lines 9-15) with movement started by a linear actuator (5, 5a).
More in detail, the articulated quadrilateral mechanism enables the grip tool to perform an insertion stroke (as shown in FIG. 5), pushing itself between the tire bead and the rim retention flange, and an identical return stroke, gripping the tire and extracting it from the rim (as shown in FIG. 6).
In this respect, it must be underlined that, sometimes, the trajectory covered by the grip tool during its insertion and its extraction can cause bothersome and excessive stress to the tire bead, with the risk of damaging it considerably and negatively affecting the safety of the wheel on the road.
In fact, to insert and extract the grip tool correctly and without wearing down the bead too much, the grip tool ought to be inserted at a specific angle and then extracted at a different angle; the presence of a mechanism with a degree of freedom such as the articulated quadrilateral mechanism envisaged in the turret, described in U.S. Pat. No. 7,128,119, however, rules out such possibility.
Another type of turret is described in patent IT 1 342 902 and comprises a support structure that can be placed resting on the rim of the wheel being machined and which has two operating levers.
The first lever is designed to be arranged astride the retention flange to keep away the bead.
The second lever, on the other hand, has a bead grip foot and is designed to be introduced between the retention flange and the bead to grip and extract the latter.
It is underlined that, unlike what happens with the grip tool of U.S. Pat. No. 7,128,119 and in the patent IT 1 342 902, the trajectory covered by the second lever during introduction into the rim channel is different from that covered during extraction.
This turret of known type is however also susceptible to further upgrading in order to make it less complex in terms of construction and operation.
Furthermore, another type of turret is known having a mobile grip tool for insertion and extraction arranged between a first and a second guiding nut with a shape substantially the same as that of the traditional work tools.
Alongside the first guiding nut, furthermore, a guiding wing is fitted.
When the turret is arranged in operating position in the proximity of the retention flange, therefore, the following elements are positioned in succession along the flange: the guiding wing, the first guiding nut, the grip tool and the second guiding nut.
It must be underlined that this turret also has a number of drawbacks tied, in particular, to the geometric shapes of the guiding wing, of the guiding nuts and of the grip tool which do not allow a correct and perfect operation to guide the tire bead to be fitted and/or removed.
At the same time, it is pointed out that, similarly to U.S. Pat. No. 7,128,119, this automated turret also envisages an articulated quadrilateral mechanism to operate the grip tool and, consequently, it is affected by the same drawbacks tied to the limited movement already described in relation to U.S. Pat. No. 7,128,119.